1. Field of the Invention
This invention relates to mooring devices for offshore structures. More particularly, it relates to connectors for joining the upper portion of a subsea tendon to a floating structure such as a tension leg platform, tension leg buoy, or the like.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A tension-leg platform (TLP) is a vertically moored floating structure normally used for the offshore production of oil or gas. A tension leg buoy (TLB) is a similar moored, floating structure whose topsides are typically comprised of unmanned equipment.
A TLP is permanently moored by means of tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. The tethers have relatively high axial stiffness (low elasticity), such that virtually all vertical motion of the platform is eliminated. This allows the production wellheads to be located on the deck of the platform (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This makes for a less expensive well completion and gives better control over the production from the oil or gas reservoir.
U.S. Pat. No. 7,287,935 describes a tendon assembly that is secured to a buoyant platform and extends down to a counterweight located near the lower end. A piling is embedded in the sea floor. A socket at the counterweight telescopingly slides over the upper end of the piling to prevent lateral movement of the platform. A dampening chamber is located between the socket and the piling. Ports for the chamber are arranged to allow a faster downward movement of the piling than upward movement.
U.S. Pat. No. 5,899,638 describes a segmented ring that forms a set of latches for a tendon top connector on a floating platform. The latches reside in a housing and have either a groove or thread profile on an upper internal surface. The grooves interface with a mating groove profile on the tendon. The latches rotationally pivot backward and forward from the outer portion of the bottom surface. When the latches are forward, the profile in the internal surface engages the mating profile of the tendon top joint. When the latches are swung out, these profiles clear and permit the top joint to pass through the connector. Retraction of the latches is provided by application of force on the inside surface of an annular extension on the outer portion of the latches. The top connector is said to allow for passive dynamic engagement of the connector to the tendon top joint. When the latches are permitted to move inward, contact is made with the mating profile on the tendon top joint. Downward movement of the tendon relative to the connector causes the passing profile of the top joint to engage the profile of the latches and engage the latches into the top joint. Upward movement of the tendon relative to the connector causes the latches to be pivoted out of the way, allowing the top joint to pass through the connector unrestricted.
U.S. Pat. No. 4,491,439 describes a latch that connects tendons run from a floating platform to a socket in a foundation on the sea floor. The latch includes a latch body having a plurality of dogs disposed within and urgible outward from the latch body. A piston is disposed within the latch body above the dogs and moves downwardly when released to urge the dogs outwardly from the body into latching engagement with the socket. A trigger mechanism in the latch releases the piston when the latch body lands in the socket and contacts a trigger pin projecting upwardly from the bottom of the socket. A series of wedges are disposed on the exterior of the body and inhibit lateral movement of the body relative to the socket when the tendon is subjected to a bending load.
U.S. Pat. No. 5,020,942 describes a tension leg platform top connector for connecting a tendon to the platform. The top connector has a housing with a bore containing a conical shoulder. Several segments locate on the conical shoulder and slide between an upper retracted position to an engaged position. In the engaged position, threads on the interior of the segments engage threads formed on the exterior of the tendon. A cam plate slides the segments down when the cam plate is rotated. A guide ring mounts outward of the segments. The guide ring has fingers that engage slots in the backs of each of the segments. The fingers and vertical slots allow the segments to move axially, but prevent them from rotating relative to the guide ring. A clutch ring applies a frictional force to the guide ring to resist rotation until the segments engage the tendon threads.
U.S. Pat. No. 4,871,282 describes a tension leg floating platform for offshore drilling that has a plurality of tendons extending from the sea floor to the platform in tension. The upper connector for each tendon includes a housing with a conical shoulder located therein. A terminal segment on the upper end of each tendon extends through the housing. Dogs are carried on the shoulder of the housing, each having threads on the interior for mating threads formed on the terminal segment. A cam ring moves the dogs from an outer retracted position to an inner engaged position. The cam ring can also rotate the dogs relative to the terminal to mesh the threads of the dogs with the threads of the terminal segment.
U.S. Pat. No. RE38,458 (a reissue of U.S. Pat. No. 5,984,585) describes a vertical stab tendon bottom connector for the tendon of a tension leg platform that uses a latch ring with an outer profile to engage a mating profile in a receptacle on the sea floor. The latch ring moves radially within a housing, relative to the receptacle, between an engaged position and a retracted position. The latch ring engages the receptacle by lowering the connector into the receptacle below the mating profile and then lifting it until the latch ring locks into the receptacle. The latch ring disengages the receptacle by lowering the connector beyond a recess located below the mating profile and then lifting the connector out of the receptacle. As the connector is lifted above the recess, a retaining ring is actuated by the recess to retain the latch ring in its retracted position.
U.S. Pat. No. 6,007,275 describes an apparatus employing a stopper chain locking mechanism for tension-leg platform tendons. The mechanism comprises a chain which is attached to the tendon and a stopper for attaching the chain to the platform.
U.S. Pat. No. 6,789,981 describes a vessel that includes a riser and/or tendon tensioning construction. A connector, such as an arm or deck structure, is suspended from cables movable relative to cable guides. The connector carries two or more risers and/or tendons extending from a subsea structure to above water level. The free ends of the cables are attached to weights for exerting a tensioning force on the risers, which are substantially decoupled from pitch, roll and heave motions of the vessel.
U.S. Pat. No. 4,468,205 describes a single point mooring apparatus adapted for securing large marine tanker vessels or subsea hydrocarbon well collection and storage barges in relatively shallow water. The floating mooring buoy structure is connected to a mooring base secured to the ocean floor by a plurality of tension leg attachment members of predetermined length. The buoyancy of the floating buoy structure is controlled to produce sufficient tension in the attachment member to provide a position restoring force to the moored marine vessel. The floating buoy structure is formed by an open frame work having a conical shape to minimize impact with ice floes on the floating buoy structure and have fully submerged buoyancy tanks to minimize the effect of wave forces and tied level variations on the position restoring force.